The invention relates to a speed control apparatus for d.c. motor, and more particularly, to such apparatus which exhibits a reduced temperature dependency.
When controlling the speed of a d.c. motor, the speed of rotation of the motor may be detected by a technique as illustrated in FIG. 1. Specifically, a d.c. motor 1 to be controlled and three resistors 2, 3 and 4 are connected in a bridge configuration. Representing the equivalent internal resistance of the motor 1 by R.sub.a and the resistance of the resistors 2, 3 and 4 by R.sub.1, R.sub.2 and R.sub.3, respectively, the bridge, which is fed with a voltage V.sub.0 across its pair of terminals A and B, produces a voltage Vn across a pair of detecting terminals C and D, as follows: ##EQU1## where E.sub.M represents the back electromotive force of the motor. When the resistors R.sub.1, R.sub.2 and R.sub.3 are chosen to satisfy the bridge balance condition: EQU R.sub.1 R.sub.2 =R.sub.3 R.sub.a ( 2)
the equation (1) can be rewritten as follows: ##EQU2## where Kv represents a constant relating to the back electromotive force of the motor and N the rotational speed of the motor. Thus it is seen that the voltage Vn developed across the pair of detecting terminals is a function of the rotational speed N alone, allowing the rotational speed to be determined by a measurement of the voltage Vn. Conversely, by maintaining the voltage Vn constant, a speed control can be achieved which maintains the speed N constant.
However, when ordinary resistors such as carbon film resistors are used for the resistors 2, 3 and 4, the equation (2) may not be satisfied with a temperature change since the equivalent internal resistance R.sub.a of the motor 1 has a greater positive temperature coefficient as compared with those of other three resistors. Consequently, the first term in the equation (1) is no longer negligible, and the factor R.sub.1 /(R.sub.1 +R.sub.a) in the second term varies with temperature. Thus, the voltage Vn is expressed as follows: EQU Vn=f(T, Torque)+g(T).multidot.E.sub.M ( 4)
and thus is a function of not only the rotational speed N, but is also a function of the temperature (T) and load or torque.
This means that when three resistors having an equal temperature coefficient and the motor are used to form a bridge circuit and a speed control scheme is employed which maintains the voltage Vn constant, the rotational speed will vary with a change in the temperature and the load, in a manner as illustrated graphically in FIG. 2, thus precluding a proper speed control.
To achieve a reduced temperature dependency, an approach has been proposed which employs a special resistor for the resistor 3 having the same temperature coefficient as the equivalent resistance R.sub.a of the motor. However, the manufacture of such a special resistor having a desired temperature coefficient requires that parameters of a complex manufacturing process be determined through experience and by a trial-and-error technique. If such a resistor is produced, the temperature coefficient may change during its practical use due to heat conduction from a substrate on which the resistor is mounted or terminals connected therewith or a convection within an apparatus. To prevent such external influences, a special consideration is necessary to choose a particular packaging material, its configuration or lead wire material having a reduced thermal conduction or a mounting thereof. It will be seen that the use of such a complicated manufacturing process and the difficulty of design in the choice and the implementation of a package result in an increased cost of such resistor.